This makes it clear that this is not an attempted override of the
virtual base class's method rx::TextureImpl::setImage().

Do the same for TextureD3D::setCompressedImage().
No intended behavior change.

BUG=angleproject:1083

Change-Id: I874e4101e8138c7c5000f5966afed6a339523c02
Reviewed-on: https://chromium-review.googlesource.com/293865Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Nico Weber <thakis@chromium.org>